1. Field of the Invention
The present invention relates to a structure of a battery pack.
2. Description of Related Art
As power sources for electric motor vehicles, such as hybrid vehicles and electric vehicles, there have been used battery packs configured by connecting a number of cylindrical batteries into battery sets, and housing these battery sets in cases. In such battery sets, a difference in temperature among the cylindrical batteries causes ununiformity of the charge-discharge behavior among the batteries, so that residual capacities of particular batteries become significantly decreased, which might result in progress of deterioration of the batteries. Accordingly, it is important for battery packs to uniformly maintain the temperature among a number of cylindrical batteries housed in cases.
The ambient temperature of a battery pack installed in a vehicle is affected by influences of change in outdoor temperature and heat generated by the vehicle, and thus, for example, one surface of the battery pack has a higher temperature than that of the other surfaces thereof depending on the installation place of the battery pack in the vehicle. To cope with this, there has been proposed a method of providing a thermal insulating layer to a surface to which temperature is transferred from a vehicle so as to uniformly maintain the temperature in an inside of a battery pack (see Japanese Patent Application Publication No. 2008-140630, for example).
There has recently been proposed a method of configuring a battery set by fitting plural cylindrical batteries into a metallic plate called as a thermal diffusing plate having a small thermal resistance. In a battery set using a thermal diffusing plate, the thermal diffusing plate having a small thermal resistance facilitates heat transmission among the cylindrical batteries, thus reducing a difference in temperature among the cylindrical batteries (see Japanese Patent Application Publication No. 2014-93275, for example).
The ambient temperature of the battery pack installed in the vehicle varies in all directions; therefore, a thermal insulating layer disposed to only one surface, as described in JP 2008-140630 A, cannot effectively suppress inflow of heat from the outside to the battery pack or outflow of heat from the battery pack to the outside, which makes it difficult to attain uniformity of temperature in the inside of the pack battery in some cases. In the battery set using the thermal diffusing plate as described in JP 2014-93275 A, even if the thermal insulating layer is provided only to the surface affected by thermal influence, as described in JP 2008-140630 A, an external thermal influence might cause variation in temperature in the thermal diffusing plate having a small thermal resistance. In such a case, variation in temperature of the thermal diffusing plate causes variation in temperature among the cylindrical batteries in some cases. Furthermore, disposing a thick thermal insulating layer around the entire circumference of the battery pack for the purpose of more strictly shutting off the external thermal influence might cause a problem of increase in dimension of the battery pack.